Speakers of English perceive their language as stress-timed, i.e., as having nearly equal intervals between stressed syllables (even if unstressed syllables intervene). But this perceived isochrony is in sharp contrast with acoustic measures of interstress intervals. The proposed research explores reasons for this discrepancy. One current explanation is that temporal perception may be distorted in the language context, i.e. that stress timing is a psychoacoustic illusion. We shall test this hypothesis with computer-manipulated stimuli that shift progressively between English-like and noise-like sound sequences. A second possibility, supported by our initial research on syllable sequences, is that some event in the production of stressed syllables is controlled (rhythmically) by talkers and is recovered in perception as a regularity of stress-timing. Three questions arise: What is the nature of the production "event"? Does the phenomenon generalize to normal fluent language? Can it explain the discrepancy? As to the first question, we shall use computer-controlled synthesis to manipulate the relative timing of production events and then observe the perceptual consequences. The other questions are part of a broader hypothesis about production and perception of language: as to production, the planning of an utterance starts from a base of evenly-paced stresses; timing is then modulated to convey syntactic, semantic, affective and other information; further irregularities enter as disparaties between production events and acoustic events. As to perception, the hypothesis is that irregularities of acoustic timing are interpreted as information (syntactic, semantic, etc.) plus familiar deviations from low-level production events. What remains is the underlying rhythm, perceived as an attribute of the utterance, namely, that it is stress-timed. To collect converging evidence about this hypothesis, we shall use diverse methods involving judgments of isochrony by listeners and talkers, comparisons of English with syllable-timed languages and, especially, comparisons among sets of utterances that progressively add phonetic complexity, syntactic, and then semantic structure. The problem and expected results are important in understanding how language conveys messages, since timing adds much high-level information to the lexical message.